1. Field of the Invention
The present invention relates generally to the measurement of process characteristics of melted materials and pertains, more specifically, to the on-line measurement of such characteristics as the viscosity of polymer melts for purposes of monitoring and controlling of manufacturing processes involving molten plastics.
2. Description of the Prior Art
Rheological testing equipment has been available for a very long time in conducting laboratory measurements of certain important characteristics of polymer melts used in various manufacturing processes. Thus, such properties as viscosity and melt flow index are being measured in the laboratory with increasing accuracy. More recently, efforts have been directed toward the measurement of these characteristics on-line, during the manufacturing process itself, in order to provide constant, closer control over the quality of the melt utilized in the process. On-line measurement requires equipment which not only is relatively easy to use and maintain, but which is rugged enough to withstand the operating conditions to which the equipment will be exposed. In order to be effective, the equipment must be responsive, and must avoid disturbing the manufacturing process being monitored.
Among the more successful on-line rheometers available currently are capillary rheometers which divert a portion of the polymer melt from the main stream of molten plastic, conduct measurements on the diverted melt, and then simply purge the melt out to the atmosphere, hence called A on-line rheometer. Also, rheometers have been developed in which the diverted melt is returned to the main stream, hence called A at(or in)-line) rheometer, thereby eliminating additional steps associated with the purge stream. Both on-line and in-line rheometers usually employ a first metering pump, such as a gear pump, to feed a capillary passage with a controlled flow of the diverted melt, and in case of in-line, a second metering pump to return the diverted melt to the main stream. Pressure drop along the capillary passage is measured and the temperature of the diverted melt is closely controlled with an independent heating or cooling arrangement in order to measure viscosity, as a function of the measure of the pressure drop, to gain the information necessary to control the process.
For a more complete understanding of the state of the technology with respect to rheological testing, consider some instances of known methods and apparatus as reflected by the patent literature. U.S. Pat. No. 4,817,416 to Blanch et al. discloses a system for making in-line rheological measurements utilizing a rheometer of the type in which a first metering pump delivers diverted melt from a process main stream to a capillary passage and a second metering pump returns the diverted melt from the capillary passage to the process main stream and the viscosity of the diverted melt is measured by controlling the rate of flow of the melt to maintain constant the pressure drop between spaced apart locations along the capillary passage and measuring the temperature of the melt in the capillary passage. The measurements may be made while controlling the speed of the second metering pump independent of the speed of the first metering pump to maintain the pressure at the exit of the capillary passage essentially constant. The capillary passage is placed in close proximity to the process main stream for maintaining a relatively short residence time during which the diverted melt resides outside the process main stream so as to attain a relatively quick response to changes in the measured viscosity.
U.S. Pat. No. 4,449,395 to Kurtz et al. discloses a system for testing thermoplastic material according to which a fractional, continuous, molten and flowable sample of the material is passed as a stream to and successively through each of either a controllable pumping zone maintained at constant temperature and pressure or controllable pressure zone maintained at constant flow rate and temperature, and a die zone to form a continuous strand of the material. The viscosity of the material in the pumping and die zones is measured. A measurement of elasticity is provided by obtaining a measurement of percentage of cross-section area swell in the strand in passage from the die zone over a constant length distance downstream of a point of mark sensing. The viscosity measurement is combined with the elasticity measurement to provide fuller Theological characterization data for the material.
U.S. Pat. No. 4,403,502 to Lindt discloses a motionless viscometer and associated method for resinous materials including polymeric foams and non-resinous materials includes a reservoir section, a receiver section and an interposed tube. As the foam is expanded, the axial pressure within the connecting tube is measured as is the rate of rise of the foam within the receiver element. Shear viscosity and density as functions of time may be determined.
It was with knowledge of the foregoing that the present invention has been conceived and is now reduced to practice.
A system for providing process control information concerning a polymer melt comprises: means containing a polymer melt under pressure; means for diverting a stream of said polymer melt directly from said containing means to and through an orifice of predetermined cross sectional area wherein the flow of said diverted portion to and through said orifice is unobstructed and directly dependent on the pressure of said main polymer melt, said cross sectional area being sized to permit the flow of said diverted melt polymer therethrough absence additional independent means acting on said diverted melt polymer for increasing flow through said orifice; temperature sensing means for measuring the temperature of the diverted melt stream and producing a resultant temperature signal; pressure sensing means for measuring the pressure of the diverted melt stream and producing a resultant pressure signal, said pressure signal being directly dependent on the pressure of polymer in said main polymer melt and the predetermined size of said orifice; means of measuring the production rate of the extruder; and manufacturing control system, such as a Programmable Logic Controller (PLC), means responsive to the temperature, pressure and production rate signals for determining process characteristics of the main polymer melt.
In the present invention, the flow of the diverted steam is unobstructed and characterized by the absence of pumps or other pressure enhancing devices which might be employed to increase the pressure in the diverted stream. The orifice is sized to provide for substantially continuous flow of polymer melt therethrough with the pressure differential provided by the pressure in the main polymer melt and the pressure at the orifice outlet which is typically atmospheric pressure. Preferably the orifice opening has a diameter of about 0.05 to about 0.15 inch or if the opening is non-circular an equivalent cross-seasonal area. The means containing the polymer melt is preferably an extruder. A single Screw or twin screw extruder having a Length to Diameter ratio from bout 5 to about 60, and preferably from about 20 to about 40, a pressure in the extruder of polymer melt is greater than about 20 pounds per square inch and is typically within the range of about 600 to about 2000 pounds per square inch, more typically within the 800 to 1400 range, and an extruder diameter of about xc2xe to 16, preferably about 1 to about 8 inches are preferred features.
With the above system a passage extends between an inlet communicating with the main polymer melt and the outlet at the orifice. In the most preferred embodiment, the passage comprises a barrel member extending between inlet and outlet ends selectively attached at said inlet to said main polymer melt. The orifice comprises an insert mounted internal to the outlet end of the barrel. A heater means may be provided for maintaining substantially constant temperature of the passageway. In the situation where the orifice is located at a location remote from the extrudeand, preferably it is located within about 5 inches of the extruder, the pressure measurement means and the temperature measurement means are adjacent or at least in very close proximity to the orifice to measure tie pressure drop across the orifice and the melt temperature at that location.
In the most preferred embodiment, the present invention relates to a system for providing process control information, such as viscosity, concerning a polymer melt comprises a barrel member extending between inlet and outlet ends which is selectively attached to a conduit containing a main stream of the polymer melt. The barrel member is adapted to receive at its inlet end a diverted melt stream from the conduit and is provided with temperature and pressure sensors for measuring the temperature and pressure, respectively, of the diverted melt stream. A capillary orifice is provided at the outlet end of the barrel member for receiving, then discharging, the diverted melt stream. As discharged, the melt stream may be at least partially solidified. A system in place to measure or calculate the total production rate through the extruder. A manufacturing control system is responsive to signals received from the temperature, pressure and production rate sensors for determining the process characteristics of the main stream of the polymer melt. A heater jacket preferably surrounds the barrel member for maintaining its temperature substantially constant. The capillary orifice has an L/D which is greater than about 3:1. The diverted melt stream transforms into at least partially solidified material after it is discharged from the capillary orifice and may be dropped onto a receptacle surface which is continuously weighed.
The invention is directed to an on-Line rheometer device capable of measuring melt viscosities of glass and/or mineral filled and unfilled thermoplastic resins. Preliminary results of several different glass filled flame retardant and non flame retardant grades from a manufacturing pilot line have indicated that differences between the on-line and quality assurance laboratory measurements are within the standard deviation of the laboratory measurement itself.
In order to improve critical to quality variables such as viscosity, flow consistency, and laboratory response time, prior to the present invention, it was realized that an in-/on-line rheometer would be highly desirable. When addressing the problem which led to the invention, the expectations were: (a) the improved rheometer has to produce transformable data xe2x80x9capples to applesxe2x80x9d, from in/on-line to the quality assurance laboratory; and (b) the device has to be robust, user friendly, and simple for quick mounting, and easy maintenance.
Previously, commercially available in/on line rheometer can not meet the expectations recited in (a) and (b) above. The difficulties lay with the inability of the components in known rheometers, such as gear pumps, stress transducers, sensors and other moving parts to withstand the wear caused by abrasive fillers such as glass fibers and minerals. The goal, then, realized by the invention, was to provide a device capable of measuring the melt viscosity of unfilled, and glass and or mineral filled resins in real time that would provide xe2x80x9capples to applesxe2x80x9d comparison with quality assurance laboratory data. The resulting device is robust, easy to maintain, user friendly, and can be coupled and decoupled quickly from the die head. The melt flow through the rheometer is naturally self-cleaning and hence it does not require physical clean up between grades of polymer melt material. It is also desirable to reduce wear.
The on-line rheometer device of the invention comprises a modified die head, a capillary orifice, pressure and temperature sensors located upstream of the orifice, an option of a mini-pelletizer or grinder, a stationary or rotating receptacle, and a weight recorder. The heart of the rheometer is the capillary orifice with L/D greater than xcx9c3:1 where L and D are length and transverse dimension, perhaps diameter, respectively.
The conduit containing a main stream of the polymer melt, or possibly he die head may be modified by drilling a small hole, perhaps 0.5 inch in diameter, into the conduit to tap into the main stream thereby forming a side stream, then adding a barrel extension to the side stream with a capillary orifice threaded in or mounted at its exit. The rheometer barrel that is extended out of the die head is wrapped with a heater jacket to prevent heat loss or gain. A very small fraction of the polymer melt is diverted to the rheometer barrel and the capillary. The air-cooled plastic is then discharged. The response of the rheometer is minutes after making setpoint adjustment. To lengthen the interval required to empty the receptacle, the receptacle may be rotated, or a grinder/pelletizer may be used, or both.
Pressure and temperature transducers and a production rate output, such as feeder rates, send respective output signals (pressure, P, temperature, T; and production rate, MR, respectively) to the manufacturing control system. The manufacturing control system carries out pre-programmed tasks to convert the signals to viscosities. These tasks include execution of certain mathematical functions or algorithms derived from fundamentals of fluid mechanics and rheology.
The rheometer just described, unlike commercially available options, does not require that the pressure and temperature in the die head and in the capillary be controlled. The algorithm to calculate real tic viscosity, from P, T, and MR, is unique and the constants used in the algorithm are characteristics of the product and thus referred to as signatures. The rheometer""s capillary orifice enables xe2x80x9capples to applesxe2x80x9d comparison with quality assurance data since the melt viscosity rheometers used in the quality assurance laboratory are based on capillary geometry (for example, a melt flow indexer described in ASTM D1238). The elimination of the gear pump also greatly increases the flexibility and durability of the rheometer of the invention.
The present invention has several features and provides a number of advantages, some of which are summarized as follows: it enables truly on-line measurements for attaining quicker response and more accurate control of manufacturing processes involving polymer melts; mounts easily to a large variety of polymer processors such as single and twin screw extruders as well as any other continuous mixers and kneaders; permits the conduct of on-line measurements with a minimal intrusion and no contamination of the main process stream being monitored; permits increased versatility in the nature and extent of the information derived from on-line measurements of polymer melts, as well as increased accuracy in the information itself; enables ease of installation and use in connection with current manufacturing equipment and techniques; allows ready adaptation for use in connection with a wide variety of materials and operating conditions; provides a convenient station for additional sensing and observation devices available for monitoring the quality of the polymer melt and integration with manufacturing processes via feedback loop controller facilitates cleaning and general maintenance, as well as replacement of component parts either for repair or adaptation to specific materials and operating conditions; and provides a simple and rugged construction without moving parts for economical manufacture and reliable long term service. This data may be used as an input for the determination of the viscosity on a continuous basis. An advantage of this technique is a reduction in weight variability which might occur when using the stationary receptacle.
Other and further features, advantages, and benefits of the invention will become apparent in the following description taken in conjunction with the following drawings. It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory but are not to be restrictive of the invention. The accompanying drawings which are incorporated in and constitute a part of this invention, illustrate one of the embodiments of the invention, and together with the description, serve to explain the principles of the invention in general terms. Like numerals refer to like parts throughout the disclosure.